In telecommunications networks transferring speech, such as telephone networks and mobile communications systems, an echo may occur on end-to-end connections, as the voice of the talking party is reflected from the network elements or the connection. The echo is disturbing if there is a significant delay on the end-to-end connection. The delay is usually a propagation delay which causes the echo especially in long-distance or international calls and in calls using satellite links. The delay may also result from digital signal processing which is the main reason for echo in digital mobile communications systems. The echo is divided into two categories: an acoustic echo between the ear piece and the microphone over telephone and an electric echo caused in transmission and reception directions in communications systems. One of the main reasons behind the electric echo is hybrid circuits (two-wire-four-wire converters) located in local exchanges or remote subscribers' stages.
An echo canceller or an echo suppresser has conventionally been used to obviate problems caused by the echo. An echo canceller is a device for processing a speech signal. It estimates the echo and reduces it by subtracting the echo estimate from a signal returning from the echo path.
In mobile communications systems, the echo cancellers are typically included in the mobile terminals and mobile switching centers which provide a gateway to other network, such as the public switched telephone network PSTN. The mobile switching center providing a gateway to the PSTN is referred to as a PSTN gateway-MSC herein. In the MS-MS call both the terminals contain the needed echo canceller device and the network elements involved in the call need not have any echo canceller. In the PSTN-MS call another echo canceller must be connected to the PSTN side of the call at the PSTN gateway-MSC while the other echo canceller is in the MS, or otherwise the echo is noticed in the mobile terminal.
An intelligent network is a flexible way to introduce new services into the telecommunications network. The intelligent network consists of service control points (SCP), service switching points (SSP) and various other elements. The service switching point's (SSP) functionality is typically located in the switching center of a conventional communications network, such as the MSC or the GMSC in the GSM system. The SSP/MSC is then connected to the service control point (SCP) which contains the intelligent service programs. The call control of the MSC uses a basic call state model (BCSM) which contains various detection points (DP) for modeling the state of the call towards the SCP. One or more of the detection points may be provided with a service trigger for the desired IN service. When call establishment proceeds to the detection point in the BCSM, and there is an activated service trigger, the SSP/MSC sends a request to the SCP. The SCP responds with operations which control the functions of the SSP/MSC in such a way that the activated service can be provided.
The IN services or similar services (e.g. services initiated by a voice message service or a voice mail system VMS) may include call drop back, call drop back return, follow on call, and call party handling services. The call drop back mechanism allows subscriber A to make a call to the person who has left a message in the voice mail. The call will be “dropped” to some of the previous MSCs and rerouted to subscriber C. In this mechanism subscriber A does not have to release the original call and make a new call to subscriber C. Call drop back return is a re-establishment of the MSC-VMS/service node (VMS=Voice Mail Service) connection, when the connection to the subscriber C has been successful and when the subscriber C has hung up the phone. The call can then be forwarded again to another subscriber. The follow-on call is a feature that enables the user to make a series of service requests without going through the identification and authentication process before performing each requested service. The identification and authentication are performed only on the first service request of the series. This functionality may be enhanced with other capabilities, it may be used e.g. for the call drop back service. One further service is call party handling (CPH). The CPH operations allow to establish new call parties in a flexible manner or to remove parties from the call and control the cross-connections between the parties. All the above-mentioned services may involve change of the connection configuration during the call.
The echo problem can appear in some particular cases when the connection configuration is changed during the call. This problem arises if the original connection configuration does not reserve any echo canceller, but the new connection configuration would require one. This happens, for example, when a PSTN-PSTN connection is changed to a PSTN-MS connection. The same problem also concerns private branch exchange (PBX) calls. As an example, let us assume that a call addressed to a mobile station MS is routed from the PSTN via a PSTN gateway mobile switching center (PSTN gw-MSC) to a serving mobile switching center (MSC) within a mobile network. Then, due to an activated call-forwarding-when-busy service, for example, the call is further routed from the serving MSC via the same or another PSTN gw-MSC to the PSTN. In a PSTN-PSTN call no echo canceller is reserved in the PSTN gw-MSC. Then, later during the call, when for instance the CPH is used, the connection configuration is changed by releasing the second (forward) PSTN leg and establishing a new connection leg to the called MS under control of the serving MSC. The new PSTN-MS call would require an echo canceller but the PSTN gw-MSC has not reserved any echo canceller for the call. Therefore, the MS subscriber will experience a disturbing echo. One solution to this problem would be to provide an echo canceller in each MSC so that the serving MSC can internally connect an echo canceller to a call connection while changing the connection configuration.
Similar problem may arise when the type of call is changed during an active call. For example, a call may be started in a speech mode requiring an echo canceller in the PSTN gw-MSC and then later be changed into a facsimile mode in which the echo canceller is not allowed. Thus, also switching between the speech mode and the fax mode during a call requires an echo canceller in each MSC so that the echo canceller can be connected to and disconnected from the call.